The present invention pertains generally to the field of internal combustion engines. More specifically, the present invention pertains to a load transfer device for use in pushrod engines that supplies additional overall valve train spring pressure directly to the body of a hydraulic roller lifter in engagement with a camshaft.
Internal combustion engines are typically provided with valve mechanisms to allow an air/fuel mixture to be introduced into a combustion chamber for ignition, and after combustion to allow the resultant byproducts of combustion to be exhausted from the combustion chamber. Each cylinder in an engine is typically provided with at least one intake valve and one exhaust valve which in turn are controlled by a valve train actuation mechanism that regulates the timing as well as the amount and duration of intake and exhaust valve opening and closing events.
In a typical overhead valve engine, the valve stem of each individual valve (intake and exhaust) is positively engaged and biased in an upward direction by a compressed coil spring positioned over the valve stem. This spring arrangement causes a constant upward spring bias on the valve stem and forces the valve face or head at the opposite end of the valve into sealing engagement with a corresponding valve seat formed in a engine cylinder head. In this manner, the valve spring bias maintains the valve in a normally closed position.
In an underhead cam pushrod engine the valves are actuated by pivotally mounted rocker arms that engage the top of the valve stems on one end and which are engaged by pushrods on the opposite end. The pushrods are engaged by valve lifters or tappets which reciprocate within bores formed in the engine block. As the valve lifter rides upward on a camshaft lobe, the upward motion of the pushrod on the rocker arm forces the valve stem downward against the spring pressure of the main valve spring and causes the valve face to separate from the valve seat and move to an open position. This allows the combustion chamber to be either charged, in the case of an intake valve, or purged after the power stroke in the case of an exhaust valve.
As the camshaft rotates, the individual camshaft lobes rotate under the valve lifters to control upward and downward pushrod motion and ultimately, through the rocker arms, the valve opening and closing events. It can be appreciated from this arrangement that as a camshaft lobe rotates past the associated valve lifter that the valve spring pressure will force the pushrod (through the rocker arm), and associated valve lifter downward following the profile of the camshaft to allow the valve to return to a closed position.
The opening and closing events of the valve train are critical in determining the overall performance of any given engine. In order to maintain proper valve timing, the valve springs must be of large enough spring force to maintain the lifter body in constant contact with the rotating camshaft over the entire operating range of the engine. When the valve return spring pressure is not great enough, lifter separation occurs and uncontrolled valve train motion may result. This condition, commonly known as lifter or valve float, can result in the loss of engine power as well as the possibility of parts breakage.
It is desirable in many high performance and racing applications to operate at elevated engine revolutions per minute ("r.p.m.") levels to obtain increased power output, as engine horsepower increases are proportional to increases in r.p.m.'s. However, at elevated engine r.p.m. levels the problem of lifter separation becomes particularly acute in that the valve lifter begins to have difficulty accurately following the camshaft profile.
The ability of the valve lifter to accurately follow the camshaft profile is normally dictated by valve spring pressure, with larger valve spring pressures providing the capability of higher permissible rpm ranges without incurring uncontrolled valve train motion. Unfortunately, because the pushrod engages the interior of the valve lifter through a relatively small contact portion, the amount of spring pressure that can be exerted on the interior of the valve lifter is extremely limited. Exceeding valve return spring pressure specifications may result in the collapse of the interior mechanism of the lifter as well as the possibility of bending the pushrods, in either case, causing severe engine damage.
Performance enhancement devices variously known as helper springs or "rev kits" have been developed by aftermarket manufacturers to safely provide increased valve train stability at high r.p.m. levels for racing and high performance applications. An example of a simple helper spring is illustrated in U.S. Pat. No. 3,280,806 to Iskenderian. In this patent, a helper spring in the shape of a bail is positioned between the rocker arm and the base of the main valve spring to independently maintain the rocker arm assembly in pressure contact with the camshaft. The helper spring thus relieves the main valve spring of the burden of maintaining lifter contact with the camshaft, and allows the main valve spring to simply provide the necessary return pressure to close the valve. However, the helper spring in this patent is not designed to increase the overall valve train spring pressure or ultimate force to be applied to the valve lifter to permit high r.p.m. operation. Moreover, there is no load transfer from the pushrod to the lifter body since the spring pressure is applied to the rocker arm by the helper spring and undesirably transmitted through the pushrod to the internal mechanism of the lifter body in the same manner as the force from the main valve spring.
A more desirable known approach involves transferring spring pressure away from the pushrods and their small internal contact point inside the lifter to the lifter body which is completely rigid and therefore capable of tolerating much greater spring pressures than the internal lifter mechanism. Such increased spring pressures enable the use of much more aggressive opening and closing valve rates with respect to camshaft profiles thus enabling the engine to produce even greater r.p.m. and horse power levels. Examples of state of the art valve train spring load transfer devices are commercially known as the "Ultra Rev Kit" marketed by Isky Racing Cams and the "Rev Kit for Harley Davidson" marketed by Fueling R & D.
Roller valve lifters are another performance enhancement device commonly employed in high performance engine application. These lifters have roller bearings at the end of the lifter that rides on the rotating camshaft. The use of roller lifters is desirable as their rolling contact surface reduces lifter/camshaft friction resulting in improvements in power output as well as reductions in fuel consumption. In recognition of these improvements, roller valve lifters have been used by the racing community and by high performance engine builders for many years to obtain increased engine power output. Moreover, several major auto manufacturers have recently started supplying engines with hydraulic roller valve lifters directly from the factory to squeeze additional efficiency and power output from their engines. Examples of roller lifter assemblies are given in U.S. Pat. No. 3,301,241 to Iskenderian and in U.S. Pat. No. 3,108,580 to Crane.
Unlike conventional valve lifters, roller valve lifters must be prevented from rotating as they reciprocate in the engine bore so that the roller bearing on the roller lifter is maintained in correct alignment with the contacting surface of the camshaft. One technique for preventing roller lifter rotation involves "ganging" adjacent lifters together through use of an appropriate alignment bar affixed to adjacent lifters. Another known anti-rotation technique uses an appropriate retainer that is installed over the top of adjacent lifter bodies, and which has flat sides that mate with corresponding flat sides in the lifter body to prevent rotation.
Despite the prevalent use of hydraulic roller lifters there are presently no known load transfer devices that can be used with these lifters. The known roller lifter anti-rotation means are not compatible with existing load transfer devices such as the devices sold by Iskenderian and Fueling noted above. Accordingly, it would be desirable to provide a valve train load transfer apparatus that works with hydraulic roller lifters to supply increased valve train spring pressure directly to the rigid body of the roller lifters.